some time ago I read of a rule-of-thumb for calculating the bitrate in multichannel surround encoders, but unfortunately I've forgotten it.It describes the influence of correlation and channel coupling between surround channels on the needed bitrate at given quality. Let me explain:When you use joint-stereo with layer III, it's better quality than plain stereo, and likewise the needed bitrate for a 5 channel encoder (eg AC3) is NOT 5 times the bitrate of one mono channel of the same quality, but - let's say - 3 times or so.